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Virulence factors of Listeria monocytogenes

Problem

Listeria monocytogenes is a Gram-positive, rod-shaped bacterium responsible for listeriosis, a severe food-borne human infection with an overall mortality rate of 30%. It has evolved efficient strategies to survive in the intestine and cross the intestinal, blood-brain and placental barriers leading, to gastroenteritis, septicemia, central nervous system infections, and mother-to-child infections.
In the body, L. monocytogenes spreads from cell to cell, aided by the listerial membrane surface protein ActA, coded by the gene actA. The bacterial membrane-bound ActA protein recruits the Arp2/3 protein complex, which helps to polymerize actin filaments at the posterior end of the bacterium, creating an actin comet tail within the host cell and propelling the bacterium forward. This function allows the bacterium to move throughout the host cell cytoplasm as well as to invade neighboring eukaryotic cells.
Figure 1 ActA protein, tethered to the bacterial membrane, stimulates actin filament nucleation with the Arp2/3 complex. This generates branched arrays of filament that grow towards the membrane to which they are tethered, elongating the actin strand and moving the bacterium. Attribution: “Benjamin A Smith, Shae B Padrick, Lynda K Doolittle, Karen Daugherty-Clarke, Ivan R Corrêa Jr, Ming-Qun Xu, Bruce L Goode,Michael K Rosen, Jeff Gelles et al, CC-BY 4.0.
The danger of its contamination of refrigerated foods have drawn more attention to the growth and survival of L. monocytogenes, as well as potential mechanisms to combat infection, as described in the experiments below.
Experiment 1
Researchers studied the peculiar property of L. monocytogenes to grow and multiply at lower temperatures than other bacteria. Two different types of foodborne bacteria, L. monocytogenes and Escherichia coli, were cultured in a medium and refrigerated at 4, degreesC. The colonies for the two species were kept separate to prevent any horizontal gene transfer. The amount of bacteria was then measured every few weeks and a growth curve was determined.
Figure 2 L. monocytogenes and E. coli bacteria growth curves.
Experiment 2
Researchers studied CK-0944636, a molecular inhibitor that can be used to bind Arp2/3 within its hydrophobic core. This compound can be used to inhibit the formation of the actin filament comet tail by and is used as a way to study Arp2/3 complex in living cells.
The researchers cultured different colonies of L. monocytogenes inside media with freely soluble actin monomers, in order to study the relative effects on actin polymerization. Strain A was exposed to CK-0944636 molecular inhibitor and also had its actA gene disrupted. Strain B had only its actA gene disrupted, whereas strain C was only incubated with CK-0944636 molecular inhibitor.
Figure 3 The rates of actin filament polymerization in µ, M, start superscript, −, 1, end superscript, s, start superscript, −, 1, end superscript for three different strains of L. monocytogenes
Data adapted from: “Dortet L, Mostowy S, Louaka AS, Gouin E, Nahori M-A, et al. (2011). Recruitment of the Major Vault Protein by InlK: A Listeria monocytogenes Strategy to Avoid Autophagy. PLoS Pathog 7(8): e1002168.
The growth exhibited from weeks 0-3 in the bacterial growth curves (Figure 3) is attributed to which process?
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